Such a method is known, for example, from WO 2007/088015 A1. In the known method, a magnetic field covering at least sections of the fluid stream is generated by at least one field coil, and, via two sensor coils, each of which may be connected to an evaluation device. The presence of a particle in the fluid stream is detected from the signal induced in the respective sensor coil. If a particle flows through the section or the magnetic field covered by the field coil, the voltage induced in the respective sensor coil changes in such a way that a detection signal may be tapped and evaluated via the evaluation unit. In the known method, two sensor coils wound in opposite directions are used. As a result of those two sensor coils the sensitivity with respect to the particle to be detected is increased. For example, smaller particles having a size of 50 to 100 micrometers may also be detected.
By such methods, particles are detected in the fluid or fluid stream in a system for cooling and/or for lubricating components of a drive unit with the aid of a fluid, in particular for cooling and/or for lubricating a gear unit of a wind turbine. Maintenance of such systems in wind turbines is particularly costly due to the high mounting location of the gear unit. Detecting at an early stage particle loads caused by wear of mechanical components or by dirt entering from the outside that may particularly damage to the cooling system and/or lubricating system is therefore desirable. In addition to the particles, gas bubbles, in particular air bubbles, are sometimes also detected as particles, to be trapped in the fluid guided through the respective system. Because of the false positive identification of air bubbles as particles, early damage detection is adversely affected, since an actual rise in the particle count rate may only occur beyond the signal noise to be associated with the air bubble signals. Consequently, an excessively high particle load is sometimes indicated and potential damage is indicated prematurely.